The present invention relates to a steering system for a motor vehicle which is not trackbound and whose steered vehicle wheels are operationally connected in normal operation to a steering handle or a steering handwheel via an electronic control arrangement which continuously checks for correct functioning and controls a steering actuator, connected with the steered vehicle wheels so as to adjust them for steering, and connected for this purpose to a steering angle desired-value sensor actuated by the steering handle or the steering handwheel and to a steering angle actual-value sensor actuated by the steered vehicle wheels. The control arrangement preferably also controls a hand-force setter connected to the steering handle or the steering handwheel so as to simulate a steering resistance, and for this purpose is connected to a hand-force desired-value sensor detecting transmitted forces between the steering actuator and steered vehicle wheels, and to a hand-force actual-value sensor detecting transmitted forces between the hand-force setter and steering handwheel, in the case of abnormal operation or emergency operation are mechanically positively coupled to the steering handle or the steering handwheel for the purpose of steering adjustment by the closure of a clutch open in normal operation, in a mechanical steering gear arrangement arranged between the steering handle or steering handwheel and steered vehicle wheels.
A known steering system is described in DE 195 46 733 C1 and DE 690 22 848 T2.
Furthermore, DE 39 19 990 A1 discloses a steering system in which automatic steering corrections can be performed, for example in the case of a side wind.
DE 37 14 833 A1 relates to a power steering system with a hydraulic and electric servomotor, the electric servomotor being switched to act via a clutch in the event of failure of the hydraulic servomotor.
Steering systems which operate using the "steer by wire" concept, at least in the case of normal operation, are being developed for future motor vehicles. Such systems offer the basic advantage that they are suitable for the most diverse vehicles without design modifications, at least with regard to the control arrangement and the associated sensor system. On the one hand, it is possible by means of appropriate programming to implement virtually any transmission ratio between the stroke of the steering handle and the steering angle change of the steered vehicle wheels. It is, moreover, possible to connect the control arrangement to additional sensors in order automatically to take account of or compensate parameters, for example side wind influences, to be prescribed.
In order to be able to ensure the required measure of safety in the case of system faults, it can be provided that upon the occurrence of a fault in the control arrangement, or upon the failure of signals which are evaluated by the control arrangement, an operating mode for abnormal operation or emergency operation is automatically switched on. In this mode of operation, it is then possible to provide positive coupling between the steering handle and steered vehicle wheels, with the result that the steering system operates in principle like a conventional steering system, although the mechanical steering column customary in conventional steering systems can, if appropriate, be replaced by other mechanical systems or else by hydraulic, in particular hydrostatic, systems.
It is the object of the invention to ensure increased safety for the "steer by wire" operation.
This object is achieved according to the invention by virtue of the fact that a sensor system is present for detecting the open state of the clutch.
The possibility of the occurrence of critical operating states owing to defective closure of the clutch can thereby be avoided. In the case of a steering system of the type described above, there is regularly a different transmission ratio between the stroke of the steering handle and the steering angle change of the steered vehicle wheels with the clutch closed to with the clutch open, that is to say in the case of normal operation. If during this normal operation the clutch were to close completely or partially in an unnoticed fashion, actuating movements of the steered vehicle wheels could lead to an undesired effect on the steering angle desired-value sensor, with the result that the electronic control arrangement then attempts to undertake a corresponding adjustment of the steered vehicle wheels, the consequence then being renewed adjustment of the steering angle desired-value sensor. Substantial instances of maloperation can "escalate" as a result.
In accordance with a particularly preferred embodiment of the invention, it is provided that the steering system switches over automatically to abnormal operation or emergency operation when during normal operation a signal for the open state of the clutch vanishes and/or a signal for a closed or closing clutch occurs.
It is expedient in this regard if the clutch is continuously subjected to a closing force, that is to say continuously forced into its closed state, and can be opened against the closing force by an actuator, rendered effective only by the supply of energy.
A particularly high degree of safety is ensured in that the steering handle and steered vehicle wheels are positively coupled to one another, and abnormal operation or emergency operation is switched on as soon as the sensor system "notices" that no unambiguous signal for an open clutch is present in normal operation.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.